The UMOX is a device that has been designed as an alternative to the conventional face mask for preoxygenation to be used when an optimal preoxygenation with a facemask is difficult to achieve(i.e: when a leak is present. A previous study in our hospital (unpublished data) has shown that the UMOX was only as effective as the conventional mask when a nose clip was used to prevent the patients from breathing in some air through the nose, thus preventing the dilution/contamination of the 100% oxygen delivered.In that study, the verbal indication giving to the subjects to breathe through the mouth was better than no indication at all but still yielded unsatisfactory results.